Another common linear data structure is the stack. Just like we did with the ordered list, we will examine the abstract view of a stack first and then look at a couple of ways a stack can be implemented. In one way, a stack is very similar to a list except that a stack is more restricted. The animation below should give you a good idea of the abstract view of a stack. Follow the directions to manipulate a simple stack and learn about the operations that a stack provides.
Now that you know how a stack works, you can see that this data structure is really a restricted list. With the stack, we have restricted the access to one end of the list by using the pop and push operations. The result of this restriction is that items in the list pile one on top of the other. To get to the bottom item, we must first remove all the items above it. This behavior is sometimes described as "last-in, first-out" or LIFO since the last item to enter the stack is the first item to leave the stack. With the stack, the top item is always the last item to enter the stack and it is always the first item to leave the stack since no other items can be removed until the top item is removed.
Let's take another look at the operations that can be performed on a stack. We will represent these two operations with the following notation:
Item PushStackItem(Stack, Item)
The PushStackItem operation has two parameters which are a stack and an item. This operation adds the item to the top of the specified stack. The PopStackItem operation only takes one parameter which is a stack. However, notice that this operation has the keyword Item listed to the left. This keyword represents the item that is removed from the top of the stack when the PopStackItem operation is done. These two operations are part of the abstract view of a stack. They are what we expect from any stack regardless of how it is implemented in the computer.